Multi-piece solid golf ball

ABSTRACT

A multi-piece solid golf ball has a core, at least one intermediate layer and an outer layer. The core is formed of a single layer or of two or more layers. The interior hardness profile of the core is set in a specific range, and the relationship between the compressive deformations of the core and the golf ball under given loadings is set in a specific range. The outer layer is formed primarily of a thermoplastic polyurethane material, and has a material hardness that is set in a specific range.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball having a core, at least one intermediate layer and an outer layer. More specifically, the invention relates to a multi-piece solid golf ball which fully satisfies various performance attributes desired by golfers in a golf ball.

In numerous disclosures to date, efforts have been made to optimize the rebound and feel of a golf ball, and also the spin rate on approach shots, by closely specifying the cross-sectional hardness of the core. Such art is described in, for example, the following technical literature: JP-A 2007-152090 (and the corresponding U.S. Pat. No. 7,273,425), JP-A 2008-194473 (and the corresponding U.S. Pat. No. 7,481,722), and JP-A 2010-214105 (and the corresponding U.S. Pat. No. 7,909,710).

In addition, numerous disclosures have been made on art which, in order to obtain, for example, the feel at impact and the low spin rate on full shots that are desired, adjusts within a specific range the deformation by a golf ball when compressed under a given load. Examples of such disclosures include JP-A 8-131580, JP-A 2012-130676 and JP-A 2000-157645.

However, there is a desire for further improvements over the above art, particularly an increased distance, a better controllability on approach shots and a better durability to repeated impact. That is, given the great intensity of research and development on golf balls in recent years, in order to secure a competitive advantage with the ball, there exists a need to raise the level of the overall properties of the ball.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a multi-piece solid golf ball which satisfies the excellent feel at impact, low spin rate on full shots and controllability on approach shots desired by golfers, and which also has an excellent durability to cracking when repeatedly struck.

As a result of intensive investigations, the inventor has discovered that, in a multi-piece solid golf ball having a core, at least one intermediate layer and an outer layer formed primarily of a thermoplastic polyurethane material, by setting the difference between the compressive deformation of the core under a load of 450 kgf and the compressive deformation of the golf ball under a load of 600 kgf within a specific range, the spin rate-lowering effect on the ball when struck with a driver (W#1), a number six iron (I#6) or the like can be improved. The inventor has also found that by optimizing the cross-sectional hardness of the core, the spin rate-lowering effect can be improved even further.

Among recent golf balls in particular, three-piece solid golf balls and four-piece solid golf balls featuring a urethane cover are widely used by professional golfers and skilled amateurs. The present invention, by optimizing the internal hardness profile of the core and also the difference in compressive deformation by the core and the ball under specific loading as described above, improves not only the distance traveled by the ball on shots with a driver (W#1), but also the distance traveled on shots with middle irons such as a number six iron (I#6). Moreover, the invention provides a ball which, along with being able to achieve an increased distance and a wind-resistant trajectory due to an improved spin rate-lowering effect, has a satisfactory controllability on approach shots and an excellent durability to cracking when repeatedly struck, making it capable of withstanding harsh conditions of use.

Accordingly, the invention provides the following multi-piece solid golf ball.

[1] A multi-piece solid golf ball comprising a core, at least one intermediate layer and an outer layer, wherein the core is formed of a single layer or of two or more layers; letting Ho be a JIS-C hardness at a center of the core and Hu be a JIS-C hardness at a surface of the core, the JIS-C hardness Ho is from 40 to 70, the JIS-C hardness Hu is from 75 to 95 and Hu−Ho is at least 20; letting D (mm) be a radius of the core, the core has a hardness profile in which the hardness does not decrease toward the core surface from a position 55% of D from the core center; letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf and (B) be the compressive deformation (mm) of the golf ball when compressed under a final load of 600 kgf from an initial load state of 10 kgf, the value (A)−(B) is in the range of from −0.4 to 2.5; the outer layer is formed primary of a thermoplastic polyurethane material; and the outer layer has a Shore D material hardness of at least 40 and not more than 50. [2] The multi-piece solid golf ball of [1] wherein, letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf, and (C) be the compressive deformation (mm) of the golf ball when compressed under a final load of 450 kgf from an initial load state of 10 kgf, the value (A)−(C) is in the range of from 1 to 4.8. [3] The multi-piece solid golf ball of [1], wherein the core has a diameter of from 30 to 39.2 mm. [4] The multi-piece solid golf ball of [1], wherein the intermediate layer has a Shore D material hardness of at least 50. [5] The multi-piece solid golf ball of [1], wherein the outer layer has a thickness of from 0.3 to 1.5 mm.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in greater detail below.

The multi-piece solid golf ball of the invention, although not shown in an accompanying diagram, has an internal structure that includes a core, at least one intermediate layer, and an outer layer. The core may be a single layer or may be formed as a plurality of two or more layers. Numerous dimples are typically formed on the outside surface of the cover outer layer.

The core used in the invention is described. This core may be obtained by vulcanizing a rubber composition composed primarily of a rubber material. No particular limitation is imposed on the rubber composition. In a preferred embodiment, the core may be formed using a rubber composition containing, for example, a base rubber, a co-crosslinking agent, a crosslinking initiator, sulfur, an organosulfur compound, a metal oxide and an antioxidant.

The polybutadiene serving as the above rubber component must be one having a cis-1,4 bond content of at least 60% (here and below, “%” refers to percent by weight), preferably at least 80%, more preferably at least 90%, and most preferably at least 95%. If the cis-1,4 bond content is too low, the resilience will decrease. In addition, the polybutadiene has a 1,2-vinyl bond content of preferably not more than 2%, more preferably not more than 1.7%, and even more preferably not more than 1.5%.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably at least 30, and more preferably at least 35, with the upper limit being preferably not more than 100, and more preferably not more than 90.

The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K6300) as measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type), and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement was carried out at a temperature of 100° C.

From the standpoint of obtaining a molded and vulcanized rubber composition having a good resilience, the polybutadiene is preferably one synthesized with a rare-earth catalyst or a group VIII metal compound catalyst.

Such rare-earth catalysts are not subject to any particular limitation, although preferred use may be made of a lanthanum series rare-earth compound. Also, where necessary, an organoaluminum compound, an alumoxane, a halogen-bearing compound and a Lewis base may be used in combination with a lanthanum-series rare-earth compound. Preferred use may be made of, as the various foregoing compounds, those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a catalyst which employs any of the lanthanum series rare-earth elements neodymium, samarium and gadolinium is preferred, with the use of a neodymium catalyst being especially recommended. In such cases, a polybutadiene rubber having a high 1,4-cis bond content and a low 1,2-vinyl bond content can be obtained at an excellent polymerization activity.

The polybutadiene has a molecular weight distribution Mw/Mn (where “Mw” stands for weight-average molecular weight, and “Mn” stands for number-average molecular weight) of preferably at least 1.0, and more preferably at least 1.3. The upper limit is preferably not more than 6.0, and more preferably not more than 5.0. If Mw/Mn is too small, the workability may decrease, whereas if it is too large, the resilience may decline.

The above polybutadiene is used as the base rubber, in which case the proportion of the polybutadiene within the overall rubber is preferably at least 40 wt %, more preferably at least 60 wt %, even more preferably at least 80 wt %, and most preferably at least 90 wt %. The above polybutadiene may account for 100 wt %, preferably 98 wt % or less, and even more preferably 95 wt % or less, of the base rubber.

Examples of cis-1,4-polybutadiene rubber that may be used include the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 and BR51 available from JSR Corporation.

Rubber components other than the above-described polybutadiene may also be included in the base rubber, insofar as the objects of the invention can be achieved. Illustrative examples of such other rubber components include polybutadienes other than the above polybutadiene, and other diene rubbers, such as styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

The co-crosslinking agent is not subject to any particular limitation in this invention. Illustrative examples include unsaturated carboxylic acids, and the metal salts of unsaturated carboxylic acids. Examples of suitable unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. The metal salts of unsaturated carboxylic acids are exemplified by the above unsaturated carboxylic acids which have been neutralized with a desired metal ion. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred. The content of these unsaturated carboxylic acids and/or metal salts thereof per 100 parts by weight of the base rubber is preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 20 parts by weight. The upper limit is preferably not more than 45 parts by weight, and more preferably not more than 43 parts by weight.

An organic peroxide is preferably used as the crosslinking initiator. Known organic peroxides may be used as this organic peroxide. Illustrative examples include dicumyl peroxide, 1,1-di(t-butylperoxy)cyclohexane, dibenzoyl peroxide, dilauroyl peroxide and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. These organic peroxides may be used singly or as combinations of two or more thereof. Commercial products may be used as the organic peroxide. Illustrative examples of such commercial products include those available under the trade names “Percumyl D” and “Perhexa C-40” (both from NOF Corporation), the trade names “Niper BW” and “Peroyl L” (both from NOF Corporation), and the trade name “Trigonox 29” (from Kayaku Akzo Corporation).

The amount of organic peroxide included is suitably set according to, for example, the type of organic peroxide and the molding and crosslinking conditions that are selected. Although not subject to any particular limitation, the amount included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 3 parts by weight. If too little organic peroxide is included, the feel at impact may be too soft. On the other hand, if too much is included, the feel at impact may become too hard and unpleasant.

Metal oxides that may be suitably used include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or two or more may be used in combination. The amount of metal oxide included per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit in the amount included per 100 parts by weight of the base rubber may be set to preferably not more than 200 parts by weight, more preferably not more than 150 parts by weight, and even more preferably not more than 100 parts by weight. At a filler content which is too high or too low, a proper weight and a suitable rebound may be impossible to obtain.

In the practice of the invention, an antioxidant is included in the rubber composition. For example, use may be made of a commercial product such as Nocrac NS-6, Nocrac NS-30 or Nocrac 200 (all products of Ouchi Shinko Chemical Industry Co., Ltd.). These may be used singly, or two or more may be used in combination.

The amount of antioxidant included per 100 parts by weight of the base rubber, although not subject to any particular limitation, is preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and even more preferably not more than 0.4 part by weight. If the antioxidant content is too high or too low, a suitable core hardness gradient may not be obtained, as a result of which it may not be possible to obtain a good rebound, durability, and spin rate-lowering effect on full shots.

Sulfur may be optionally included in the rubber composition. The sulfur is exemplified by the product available from Tsurumi Chemical Industry Co., Ltd. under the trade name “Sulfax-5.” The amount of sulfur included can be set to more than 0, and may be set to preferably at least 0.005 part by weight, and more preferably at least 0.01 part by weight, per 100 parts by weight of the base rubber. The upper limit in the amount of sulfur, although not subject to any particular limitation, may be set to preferably not more than 0.5 part by weight, more preferably not more than 0.4 part by weight, and even more preferably not more than 0.1 part by weight. By adding sulfur, hardness differences in the core can be increased. However, adding too much sulfur may result in undesirable effects during hot molding, such as explosion of the rubber composition, or may considerably lower the rebound.

In addition, an organosulfur compound may be included in the rubber composition so as to impart a good rebound. The inclusion of, for example, thiophenols, thionaphthols, halogenated thiophenols, or metal salts thereof as the organosulfur compound is recommended. Illustrative examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, and the zinc salt of pentachlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

The amount of the organosulfur compound included per 100 parts by weight of the base rubber is at least 0.05 part by weight, preferably at least 0.07 part by weight, and more preferably at least 0.1 part by weight. The upper limit is not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Including too much organosulfur compound may excessively lower the hardness, whereas including too little is unlikely to improve the rebound.

The core can be produced by vulcanizing and curing the rubber composition containing the various above ingredients. For example, production may be carried out by using a mixing apparatus such as a Banbury mixer or a roll mill to mix the ingredients, carrying out compression molding or injection molding using a core-forming mold, then suitably heating, and thereby curing, the molded body at a temperature sufficient for the organic peroxide and the co-crosslinking agent to act, such as from about 100° C. to about 200° C. for a period of 10 to 40 minutes.

The core diameter, although not subject to any particular limitation, is preferably at least 30 mm, and more preferably at least 33 mm. The upper limit is preferably not more than 39.2 mm, and more preferably not more than 38 mm. At a core diameter outside of this range, the ball's durability to cracking may dramatically decline, or the initial velocity of the ball may decrease.

In the present invention, letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf and (B) be the compressive deformation (mm) of the golf ball when compressed under a final load of 600 kg from an initial load state of 10 kgf, it is critical for the value (A)−(B) to be in the range of from −0.4 to 2.5. The compressive deformation of the golf ball when struck with a W#1 is assumed to be similar to the compressive deformation (B) of the golf ball when subjected to a load of 600 kgf; a larger value for (B) means that the ball deformation when struck will be larger. Likewise, the compressive deformation of the core when struck under the same conditions is assumed to be similar to the compressive deformation of the core when subjected to a load of 450 kgf; the larger this value, the larger the ball deformation when struck. By making the difference between (A) and (B) above, i.e., the value (A)−(B), larger, the launch angle is increased, enabling the spin rate to be reduced. By having the deformation at impact be a suitable value, a better feel can be provided.

The above value (A)−(B) is in the range of from −0.4 to 2.5, preferably from −0.2 to 2.2, and more preferably from 0 to 2.0. In cases where this value does not satisfy the above numerical range, the spin rate is not reduced and the ball has a poor feel at impact.

In addition, letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf and (C) be the compressive deformation (mm) of the golf ball when compressed under a final load of 450 kg from an initial load state of 10 kgf, it is preferable for the value (A)−(C) to be in the range of from 1 to 4.8. The compressive deformation of the golf ball when struck with a I#6 is assumed to be similar to the compressive deformation (C) of the golf ball when subjected to a load of 450 kgf; a larger value for (C) means that the ball deformation when struck will be larger. By making the difference between (A) and (C) above, i.e., the value (A)−(C), larger, the spin rate on shots with a middle iron can be reduced and the wind resistance of the ball in flight can be increased.

The above value (A)−(C) is preferably from 1 to 4.8, more preferably from 1.4 to 4.6, and even more preferably from 1.8 to 4.4. In cases where this value does not satisfy the above numerical range, the spin rate may not be reduced and the ball may have a poor feel at impact.

As mentioned above, the core is formed of a single layer or a plurality of layers. The core diameter should satisfy the value indicated above when the core is formed of a single layer. In cases where the core is formed of a plurality of layers, the core diameter may satisfy the above value as a sum of the plurality of layers. For example, when the core is formed of two layers—an inner core layer and an outer core layer, the diameters of these respective layers are not particularly limited, although the diameter of the inner core layer is preferably from 10 to 30 mm, more preferably from 12 to 28 mm, and even more preferably from 14 to 26 mm. Outside of this range, the spin rate of the ball on full shots may not decrease. Moreover, the feel of the ball at impact may worsen.

Next, the cross-sectional hardness of the core is described.

In the invention, letting D (mm) be the radius of the core, O be the center of the core, P be a position 25% of D from the core center, Q be a position 55% of D from the core center, R be a position 65% of D from the core center, S be a position 85% of D from the core center, and U be a position 100% of D from the core center, the JIS-C hardnesses at these respective positions are designated as Ho, Hp, Hq, Hr, Hs and Hu. In the invention, Ho has a JIS-C hardness of from 40 to 70, Hu has a JIS-C hardness of from 75 to 95, and Hu−Ho is at least 20.

The JIS-C hardness Ho at the center of the core has a lower limit of at least 40 and has an upper limit of not more than 70, preferably not more than 65, more preferably not more than 63, and even more preferably not more than 60. The JIS-C hardness Hu at the core surface has a lower limit of at least 75, preferably at least 77, more preferably at least 80, and even more preferably at least 85. The upper limit is not more than 95, preferably not more than 93, and more preferably not more than 91.

The value Hu−Ho, i.e., the difference between the JIS-C hardness at the core surface and the JIS-C hardness at the core center, has a lower limit of at least 20, preferably at least 22, more preferably at least 25, and even more preferably at least 28. The upper limit is preferably not more than 50. If this value is too large, the initial velocity may be inadequate or the durability may worsen. On the other hand, if this value is too small, the spin rate of the ball may rise excessively, as a result of which the distance may be less than satisfactory, or the feel at impact may become hard.

Also, the value Hr−Hq (the hardness difference between the positions at 55% of D and at 65% of D), although not particularly limited, is preferably at least 0 and preferably not more than 25. When this value falls outside of the foregoing range, a sufficient spin rate-lowering effect may not be obtained and the desired distance may not be achieved. Moreover, the durability may worsen.

The value Hq−Hp (the hardness difference between the positions at 25% of D and at 55% of D), although not particularly limited, is preferably at least 0, and more preferably at least 3. The upper limit is preferably not more than 30, and more preferably not more than 28. When this value falls outside of the foregoing range, a sufficient spin rate-lowering effect may not be obtained and the desired distance may not be achieved. Moreover, the durability may worsen.

The value Hs−Hq (the hardness difference between the positions at 55% of D and at 85% of D), although not particularly limited, is preferably at least 5, and more preferably at least 8. The upper limit is preferably not more than 40, and more preferably not more than 37. When this value falls outside of the foregoing range, a sufficient spin rate-lowering effect may not be obtained and the desired distance may not be achieved. Moreover, the durability may worsen. Also, in this invention, it is critical for the core to have a hardness profile in which the hardness does not decrease toward the core surface from a position 55% of D from the core center.

No particular limitation is imposed on the method for adjusting the cross-sectional hardness so as to have the core satisfy the above formulas, although a core having the desired cross-sectional hardness can be obtained by suitably adjusting the core rubber formulation and the vulcanization temperature and time.

Next, the intermediate layer is described. The intermediate layer material is not particularly limited; suitable use may be made of, for example, known ionomeric resins, thermoplastic elastomers and thermoset elastomers. Examples of thermoplastic elastomers include polyester-type, polyamide-type, polyurethane-type, olefin-type and styrene-type thermoplastic elastomers. It is especially preferable to use an ionomeric resin as the base resin in the intermediate layer material. When including an ionomeric resin composition in such a case, it is desirable to use a mixture of a zinc ion (Zn²⁺)-neutralized ionomer resin and a sodium ion (Na+)-neutralized ionomer resin. The mixing ratio therebetween, expressed as the weight ratio (zinc ion (Zn²⁺)-neutralized ionomeric resin (I))/(sodium ion (Na+)-neutralized ionomeric resin (II)), is preferably between 25/75 and 75/25, more preferably between 35/65 and 65/35, and even more preferably between 45/55 and 55/45. If the resin ratio (I)/(II) does not satisfy the above range, there is a possibility that the rebound of the ball as a whole will be small, as a result of which the desired flight performance may not be achieved. Also, the durability to cracking when repeatedly struck at ordinary temperatures may worsen, and the durability to cracking at low (subzero Celsius) temperatures may worsen.

Also, it is desirable to carry out abrasion treatment on the surface of the intermediate layer so as to increase adhesion with the outer layer located on the outside thereof. In addition, following such abrasion treatment, a primer may be applied to the surface. It is also possible to increase adhesion by adding an adhesion reinforcing agent to the intermediate layer material.

In the above case, the intermediate layer has a Shore D material hardness which, although not particularly limited, is preferably at least 50, more preferably at least 55, and even more preferably at least 60. The upper limit is preferably not more than 70, more preferably not more than 66, and even more preferably not more than 63. If the material hardness of the intermediate layer is too low, on full shots, the ball may take on too much spin, possibly resulting in a less than satisfactory distance. On the other hand, if the material hardness of the intermediate layer is too high, the durability to cracking when repeatedly struck may worsen, or the feel of the ball on shots with a putter and on approach shots may become too hard.

The thickness of the intermediate layer, although not particularly limited, is preferably at least 0.5 mm, more preferably at least 0.7 mm, and even more preferably at least 0.9 mm. The upper limit is preferably not more than 2.1 mm, more preferably not more than 1.9 mm, and even more preferably not more than 1.7 mm. If the intermediate layer thickness is greater than the above range, the spin rate-lowering effect on shots with a W#1 may be inadequate and a sufficient distance may not be achieved. On the other hand, if the intermediate layer is too thin, the durability to cracking when repeatedly struck and the durability at low temperatures may worsen.

Next, the outer layer used in the present invention is described. The outer layer in this invention is formed primarily of a thermoplastic polyurethane material for reasons having to do with controllability and scuff resistance. The use of a thermoplastic polyurethane elastomer in particular is preferred from the standpoint of amenability to mass production.

In cases where the outer layer material is a thermoplastic polyurethane elastomer, it is preferable to use one type of resin pellet composed of a resin blend in which the main components are (A) a thermoplastic polyurethane and (B) a polyisocyanate compound and, when the resin pellets are charged into an injection molding machine just prior to injection molding, it is preferable for at least some isocyanate compound to be present in which all the isocyanate groups on the molecule remain in an unreacted state. Golf balls composed of such thermoplastic polyurethane elastomers have an excellent rebound, spin performance and scuff resistance.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the outer layer-forming resin material. Specifically, it is recommended that the total weight of components A and B combined be preferably at least 60%, and more preferably at least 70%, of the overall weight of the outer layer. Above components A and B are described in detail below.

In describing the thermoplastic polyurethane (A), the structure of this thermoplastic polyurethane includes soft segments composed of a polymeric polyol that is a long-chain polyol (polymeric glycol), and hard segments composed of a chain extender and a polyisocyanate compound. Here, the long-chain polyol serving as a starting material is not subject to any particular limitation, and may be any that is used in the prior art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. The polyether polyol may be used singly or as a combination of two or more thereof. Of the above, poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-average molecular weight in the range of 1,500 to 5,000. By using a long-chain polyol having such a number-average molecular weight, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in the range of 1,700 to 4,000, and even more preferably in the range of 1,900 to 3,000.

The number-average molecular weight of the long-chain polyol refers here to the number-average molecular weight computed based on the hydroxyl number measured in accordance with JIS K-1557.

Chain extenders that may be suitably used include those employed in the prior art relating to thermoplastic polyurethanes. For example, low-molecular-weight compounds which have a molecular weight of 400 or less and bear on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diols having 2 to 12 carbons are preferred, and 1,4-butylene glycol is more preferred.

The polyisocyanate compound is not subject to any particular limitation; preferred use may be made of one that is used in the prior art relating to thermoplastic polyurethanes. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component A to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be a polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic diisocyanate to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction may be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing the thermoplastic polyurethane used as component A. Production may be carried out by either a prepolymer process or a one-shot process which uses a long-chain polyol, a chain extender and a polyisocyanate compound and employs a known urethane-forming reaction. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is also possible to use a commercially available product as the thermoplastic polyurethane serving as component A. Illustrative examples include Pandex T8295, Pandex T8290, Pandex T8283 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.).

Next, various types of isocyanates may be employed without particular limitation as the polyisocyanate compound serving as component B. Illustrative examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, the use of 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferable in terms of the balance between the influence on moldability of, e.g., the rise in viscosity accompanying the reaction with the thermoplastic polyurethane serving as component A and the physical properties of the outer layer material of the resulting golf ball.

A thermoplastic elastomer (component C) other than the above-described thermoplastic polyurethane may be included as an optional component together with components A and B. By including this component C in the above resin blend, the flow properties of the resin blend can be further increased and improvements can be made in various properties required of the outer layer material of a golf ball, such as resilience and scuff resistance.

The thermoplastic elastomer other than the above thermoplastic polyurethane which is used as component C may be of one, two or more types selected from among polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. In particular, because they increase the resilience and scuff resistance due to reaction with the isocyanate groups while at the same time maintaining a good productivity, the use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred.

The above components A, B and C have a compositional ratio, expressed as a weight ratio, which, although not subject to any particular limitation, is set to A:B:C=100:2 to 50:0 to 50.

In the present invention, when the resin blend is prepared by mixing together component A, component B and, additionally, component C, it is essential to select conditions such that, of the polyisocyanate compound, there exists at least some portion in which all the isocyanate groups remain in an unreacted state. For example, a step such as mixture in an inert gas such as nitrogen or in a vacuum state must be taken. The resin blend is then injection-molded around a core that has been placed in a mold. For easy and trouble-free handling, it is preferable to form the resin blend into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. Isocyanate groups in an unreacted state remain within these resin pellets; while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter, the unreacted isocyanate groups react with component A and component C to form a crosslinked material.

The outer layer may be molded by a method which involves, for example, feeding the above-described resin blend to an injection-molding machine, and injecting the molten resin blend over the core. In this case, the molding temperature varies depending on the type of thermoplastic polyurethane, but is preferably in the range of 150 to 250° C.

When injection molding is carried out, it is desirable, though not essential, to carry out such molding in a low-humidity environment by subjecting some or all places on the resin paths from the resin feed area to the mold interior to purging with an inert gas such as nitrogen or a low-moisture gas such as low dew-point dry air, or to vacuum treatment. Preferred, non-limiting, examples of the medium used for transporting the resin under applied pressure include low-moisture gases such as low dew-point dry air or nitrogen gas. By carrying out molding in such a low-humidity environment, the progression of reactions by isocyanate groups before the resin blend is charged into the mold interior is suppressed. Polyisocyanate in which, to some degree, isocyanate groups are present in an unreacted state is thus included within the molded resin material, making it possible to reduce variable factors such as an undesirable rise in viscosity and also enabling the real crosslinking efficiency to be increased.

Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the resin blend (outer layer material) which may be used in this invention is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10° C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150° C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observed from about 150° C., but a weight drop can be confirmed from about 230 to 240° C.

After the resin blend has been molded as described above, the properties as a golf ball outer layer can be additionally improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. “Annealing,” as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

In addition to the above-described resin components, various additives may be optionally included in the outer layer material in the invention. Examples of such additives include pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, parting agents, plasticizers, and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide, tungsten).

Next, the thickness of the outer layer in this invention, although not particularly limited, is preferably at least 0.3 mm, more preferably at least 0.4 mm, and even more preferably at least 0.5 mm. The maximum thickness is preferably not more than 1.5 mm, more preferably not more than 1.0 mm, and even more preferably not more than 0.8 mm. If the outer layer is thicker than the above range, the rebound on W#1 shots may be inadequate or the spin rate may increase, possibly resulting in a poor distance. If the outer layer is thinner than the above range, the scuff resistance may worsen, or the controllability even by professional golfers and skilled amateurs may be inadequate.

The material hardness of the outer layer, expressed as the Shore D hardness, although not particularly limited, is preferably at least 40, and more preferably at least 42. The maximum value is preferably not more than 50, more preferably not more than 48, and even more preferably not more than 46. At a low Shore D hardness, the ball may be too receptive to spin on full shots, possibly resulting in a poor distance. At a Shore D hardness which is too high, the ball may not be receptive to spin on approach shots, which may result in a poor controllability even by professional golfers and skilled amateurs.

In the golf ball of the invention, numerous dimples are provided on the surface of the outer layer for the sake of aerodynamic performance. The number of dimples formed on the outer layer surface is not subject to any particular limitation. However, to enhance the aerodynamic performance of the ball and increase the distance traveled by the ball, the number of dimples is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The maximum number of dimples is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.

The method of manufacturing multi-piece solid golf balls in which the above-described core, intermediate layer and outer layer are each formed as successive layers is not subject to any particular limitation. Production may be carried out by an ordinary method such as a known injection molding process. By way of illustration, first a core is placed within a given injection mold, following which the intermediate layer material is injection-molded over the core to form an intermediate sphere. Next, this intermediate sphere is placed in another injection mold and the outer layer material is injection-molded over the sphere, concurrent with which dimples are molded in the outer layer surface, thereby giving a multi-piece golf ball. Alternatively, instead of the above method in which the materials for the respective layers are injection-molded, use may made of a method in which the intermediate sphere is enclosed by two half-cups that have been molded beforehand into hemispherical shapes, and the resulting assembly is molded under applied heat and pressure.

The golf ball of the invention has a diameter of not less than 42 mm, preferably not less than 42.3 mm, and more preferably not less than 42.6 mm. The upper limit in the diameter is not more than 44 mm, preferably not more than 43.8 mm, more preferably not more than 43.5 mm, and even more preferably not more than 43 mm.

The weight of the golf ball is preferably not less than 44.5 g, more preferably not less than 44.7 g, even more preferably not less than 45.1 g, and most preferably not less than 45.2 g. The upper limit in the weight is preferably not more than 47.0 g, more preferably not more than 46.5 g, and even more preferably not more than 46.0 g.

As described above, the multi-piece solid golf ball of this invention has an improved low-spin performance on full shots, achieves an increased distance and a wind-resistant trajectory, and is capable of having a good feel at impact that is satisfactory to the golfer. Moreover, the multi-piece solid golf ball of the invention has a good controllability on approach shots and an excellent durability to cracking when repeatedly struck.

Examples

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 6, Comparative Examples 1 to 9

Golf ball cores were produced by using the rubber formulations in the respective Examples of the invention and Comparative Examples as shown in Table 1 below to prepare core compositions, then molding and vulcanizing the core compositions under the vulcanization conditions in the table. First, the inner core layer was molded and vulcanized. This was covered by the outer core layer material in an unvulcanized state, then molding and vulcanization were again carried out, thereby producing the core.

TABLE 1 Core formulation (parts by weight) A B C D E F G H I J K L N O Polybutadiene A 80 100 100 Polybutadiene C 20 20 20 20 20 20 20 20 20 20 20 Polybutadiene B 20 80 80 80 80 80 80 80 80 80 80 80 Peroxide (1) 1.5 0.3 3 Peroxide (2) 1.2 1.2 1.2 1.2 5 1.2 5 1.2 0.3 1.2 1.2 1.2 Barium sulfate 15.1 16.2 17.3 18 17.3 13.3 13 25.1 26.9 22.8 28.6 Zinc oxide 16.2 4 4 4 4 4 4 4 4 18.6 14.9 4 4 4 Antioxidant (1) 0.2 0.3 0.3 0.1 0.1 Antioxidant (2) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc acrylate 43 36 33.5 31 29.5 28 40.1 38 13 37 46 9 18.5 5 Zinc stearate 5 5 5 Sulfur 0.1 0.1 Zinc salt of 1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.6 1.5 0.1 0.1 0.1 pentachloro- thiophenol Vulcanization 155 155 155 155 155 155 155 155 155 160 160 155 155 155 temperature (° C.) Vulcanization 21 13 13 13 13 13 13 13 13 13 13 13 13 13 time (minutes) Details on the above materials are given below. Polybutadiene A: Available under the trade name “BR730” from JSR Corporation Polybutadiene B: Available under the trade name “BR51” from JSR Corporation Polybutadiene C: Available under the trade name “BR01” from JSR Corporation Peroxide (1): Dicumyl peroxide, available under the trade name “Percumyl D” from NOF Corporation Peroxide (2): A mixture of 1,1-di(t-butylperoxy)cyclohexane and silica, available under the trade name “Perhexa C-40” from NOF Corporation Barium sulfate: Available as “Precipitated Barium Sulfate 300” from Sakai Chemical Co., Ltd. Antioxidant (1): Available under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical Industry Co., Ltd. Antioxidant (2): Available under the trade name “Nocrac 200” from Ouchi Shinko Chemical Industry Co., Ltd. Zinc stearate: Available under the trade name “Zinc Stearate G” from NOF Corporation Sulfur: Available under the trade name “Sulfax-5” from Tsurumi Chemical Industry Co., Ltd.

Next, using the respective resin materials shown in Table 2, an intermediate layer and an outer layer were formed in this order over the core by injection molding. Although not shown in an accompanying diagram, a common dimple pattern was used in the working Examples of the invention and the Comparative Examples. The dimples were formed, during injection molding of the outer layer, by impression with numerous outer layer-forming protrusions provided on the spherical surface of the mold cavity.

TABLE 2 (parts by weight) No. 1 No. 2 No. 3 Himilan 1605 50 Himilan 1557 15 Himilan 1706 35 Trimethylolpropane 1.1 Hytrel 4001 15 T-8295 75 T-8290 37.5 25 T-8283 62.5 Titanium oxide 3.8 3.5 Polyethylene wax 1.4 1.5 Isocyanate compound 7.5 7.5 Numbers in the table indicate parts by weight. Details on the above materials are given below. Himilan: Ionomers available from DuPont-Mitsui Polychemicals Co., Ltd. Hytrel: A polyester elastomer available from DuPont-Toray Co., Ltd. T8925, T8290, T8283: MDI-PTMG type thermoplastic polyurethanes available under the trade name “Pandex” from DIC Bayer Polymer Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical Industries, Ltd. Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

Physical properties such as hardnesses of the individual layers and deformation of the ball, flight performance (carry and backspin rate), both on shots with a W#1 and on shots with an I#6, spin performance on approach shots, feel at impact, and durability when repeatedly struck were evaluated according to the criteria described below for the golf balls obtained in each of the working Examples of the invention and the Comparative Examples. The results are presented in Table 3 (Examples of invention) and Table 4 (Comparative Examples).

(1) Compressive Deformation (A) of Core Under 450 kgf Load (mm)

The compressive deformation (mm) of the core when subjected, at a temperature of 23±1° C. and a rate of 500 mm/min, to a final load of 4,410 N (450 kgf) from an initial load state of 98 N (10 kgf) was measured.

(2) Compressive Deformation (B) of Ball Under 600 kgf Load (mm)

The compressive deformation (mm) of the golf ball when subjected, at a temperature of 23±1° C. and a rate of 500 mm/min, to a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) was measured.

(3) Compressive Deformation (C) of Ball Under 450 kgf Load (mm)

The compressive deformation (mm) of the ball when subjected, at a temperature of 23±1° C. and a rate of 500 mm/min, to a final load of 4,410 N (450 kgf) from an initial load state of 98 N (10 kgf) was measured.

(4) Hardness at Center of Core (Ho)

The core was cut into hemispheres so that the cut face formed a flat plane, following which a durometer indenter was pressed perpendicularly against the center portion thereof and measurement was carried out. The JIS-C (JIS K6301-1975 standard, defined similarly below) hardness value is indicated.

(5) Hardness at Surface of Core (Hu)

A durometer was set perpendicularly against a surface portion of the spherical core, and the hardness was measured based on the JIS-C hardness standard. The JIS-C hardness value is indicated.

(6) Cross-Sectional Hardnesses of Core

The core was cut with a fine cutter. Letting D (mm) be the radius of the core, P be a position 25% of D from the core center, Q be a position 55% of D from the core center, R be a position 65% of D from the core center and S be a position 85% of D from the core center, the JIS-C hardnesses at these respective positions were designated as Hp, Hq, Hr and Hs. The JIS-C hardnesses Hp, Hq, Hr and Hs at these respective places on the core cross-section were measured.

(7) Material Hardness of Intermediate Layer

The resin material for the intermediate layer was formed into a sheet having a thickness of 2 mm, and the hardness was measured with a type D durometer in accordance with ASTM-D2240.

(8) Material Hardness of Outer Layer

The method of measurement was the same as in (7) above.

(9) Flight Test

The carry (m) of the ball when struck at a head speed (HS) of 50 m/s with, as the driver (W#1), a TOURSTAGE X-DRIVE 703 (loft angle, 8.5°; manufactured by Bridgestone Sports Co., Ltd.) mounted on a swing robot was measured. The results were rated according to the criteria shown below. The backspin rate was the value measured for the ball, immediately after impact, with an apparatus for measuring initial conditions.

-   -   Good: Carry was 245 m or more, and backspin rate was less than         2,600 rpm     -   NG: Carry was less than 245 m, and backspin rate was 2,600 rpm         or more     -   NG: Carry was 245 m or more, and backspin rate was 2,600 rpm or         more         (Note: Even when a carry of 245 m or more was obtained, at a         backspin rate of more than 2,600 rpm, the ball rises too steeply         and has a trajectory that is readily affected by the wind, in         addition to which the run is shorter, resulting in a total         distance that is less than satisfactory. Hence, in such cases,         as indicated above, the flight performance is rated as “NG.”)

(10) Middle Iron (I#6)

The carry (m) of the ball when struck at a head speed of 44 m/s with, as the middle iron, an X-BLADE CB (a number six iron manufactured by Bridgestone Sports Co., Ltd.) was measured. The results were rated according to the following criteria. The backspin rate was the value measured for the ball, immediately after impact, with an apparatus for measuring initial conditions.

-   -   Good: Carry was 157 m or more, and backspin rate was less than         6,100 rpm     -   NG: Carry was less than 157 m, and backspin rate was 6,100 rpm         or more     -   NG: Carry was 157 m or more, and backspin rate was 6,100 rpm or         more         (Note: Even when a carry of 157 m or more was obtained, at a         backspin rate of more than 6,100 rpm, the ball rises too steeply         and has a trajectory that is readily affected by the wind, in         addition to which the run is shorter, resulting in a total         distance that is less than satisfactory. Hence, in such cases,         as indicated above, the flight performance is rated as “NG.”)         (11) Approach Shots with a Sand Wedge

A sand wedge (SW) was mounted on a golf swing robot, and the backspin rate (rpm) when the ball was struck at a head speed (HS) of 21 m/s was measured. The club used was a TourStage X-WEDGE manufactured by Bridgestone Sports Co., Ltd. (2011 model; loft, 56°). The flight performance was rated according to the following criteria.

-   -   Good: The backspin rate on approach shots was 6,000 rpm or more     -   NG: The backspin rate on approach shots was less than 6,000 rpm

(12) Durability on Repeated Impact

The durability of the golf ball was evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). This tester functions so as to fire a golf ball pneumatically and cause it to repeatedly strike two metal plates arranged in parallel. The incident velocity against the metal plates was set at 43 m/s. The number of shots required for the golf ball to crack was measured. The results were rated according to the following criteria.

-   -   Good: Number of shots until cracking occurred was 150 or more     -   NG: Number of shots until cracking occurred was less than 150

(13) Feel

The feel of the ball when hit with a driver (W#1) by ten golfers was sensory evaluated under the following criteria.

-   -   Good: At least four out of the ten golfers rated the ball as         having a good feel     -   NG: Three or fewer of the ten golfers rated the ball as having a         good feel         (A “good feel” refers to a feel having a suitably soft touch; a         feel which is too soft or too hard is a bad feel.)

In addition, the feel on shots with an iron was sensory evaluated under the following criteria.

-   -   Good: At least four out of the ten golfers rated the ball as         having a good feel     -   NG: Three or fewer of the ten golfers rated the ball as having a         good feel

TABLE 3 Example 1 2 3 4 5 6 Ball (B) Compressive deformation (mm) 9.2 9.7 9.2 10.2 9.3 9.6 at 600 kgf (C) Compressive deformation (mm) 7.3 7.5 7.3 7.9 7.4 7.5 at 450 kgf Outer Material No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 layer Thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 Material hardness (Shore D) 44 44 44 44 44 44 Intermediate Material No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 layer Thickness (mm) 1.7 1.7 1.7 1.7 1.7 1.7 Material hardness (Shore D) 62 62 62 62 62 62 Core Formulation L N L L L (inner layer) Diameter (mm) 23 23 25 15 18 Weight (g) 7.5 7.5 9.6 9.6 9.6 Core Formulation A B C B D D (outer layer) Overall Diameter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 core Weight (g) 32.8 32.8 32.8 32.8 32.8 32.8 (A) Compressive deformation (mm) 9.5 10.7 9.4 11.7 9.7 10.3 at 450 kgf Hardness  0% of radius (core center) Ho 59 40 54 40 42 40 profile  25% of radius Hp 61 41 56 41 46 42 at  55% of radius Hq 64 46 64 44 71 70 core  65% of radius Hr 66 71 71 48 71 71 interior  85% of radius Hs 79 82 79 81 79 79 100% of radius (core surface) Hu 88 90 87 90 87 87 Hr − Hq (hardness difference between 2 25 7 4 0 1 55% and 65% positions) Hu − Ho (hardness difference between 29 50 33 50 45 47 center and surface) Hq − Hp (hardness difference between 3 5 8 3 25 28 25% and 55% positions) Hs − Hq (hardness difference between 15 36 15 37 8 9 55% and 85% positions) (A) − (B) compressive deformation difference (mm) 0.4 1.1 0.2 1.5 0.4 0.8 (A) − (C) compressive deformation difference (mm) 2.2 3.2 2.1 3.8 2.3 2.8 Flight Initial velocity (m/s) 73 72 73 71 72 72 on shots Spin rate (rpm) 2554 2427 2487 2429 2481 2360 with W#1 Launch angle (°) 11 11.3 11.2 11.2 11.3 11.4 (HS, 50 m/s) Carry (m) 250 246 247 245 247 247 Rating good good good good good good Flight Initial velocity (m/s) 55 55 55 55 55 55 on shots Spin rate (rpm) 6098 5818 5639 5645 5884 5719 with I#6 Carry (m) 159 157 158 158 157 158 (HS, 44 m/s) Rating good good good good good good Spin rate Spin rate (rpm) 6242 6282 6290 6265 6268 6220 on approach Rating good good good good good good shots (HS, 21 m/s) Durability Rating good good good good good good to impact Feel When struck with driver good good good good good good at impact When struck with iron good good good good good good

TABLE 4 Comparative Example 1 2 3 4 5 6 7 8 9 Ball (B) Compressive deformation (mm) 9 9 8.5 6 6.1 16.2 16.1 8 8.9 at 600 kgf (C) Compressive deformation (mm) 7.1 7.1 6.8 4.7 4.9 12.5 12.4 6.4 7.1 at 450 kgf Outer Material No. 3 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 layer Thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Material hardness (Shore D) 54 44 44 44 44 44 44 44 44 Intermediate Material No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 layer Thickness (mm) 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 Material hardness (Shore D) 62 62 62 62 62 62 62 62 62 Core Formulation ∘ ∘ (inner layer) Diameter (mm) 25 23 Weight (g) 9.6 7.5 Core Formulation E E F G H I I J K (outer layer) Overall Diameter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 core Weight (g) 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 (A) Compressive deformation (mm) 8.9 8.9 8.5 5.6 5.6 19.9 19.7 7.9 8.9 at 450 kgf Hardness  0% of radius (core center) Ho 61 61 65 70 74 32 32 71 59 profile  25% of radius Hp 67 67 71 80 82 32 32 79 72 at  55% of radius Hq 69 69 69 82 83 36 36 80 78 core  65% of radius Hr 68 68 68 81 77 40 46 83 77 interior  85% of radius Hs 71 71 68 87 86 58 58 88 79 100% of radius (core surface) Hu 84 84 84 94 98 59 59 90 93 Hr − Hq (hardness difference −1 −1 −1 −1 −6 4 10 3 −1 between 55% and 65% positions) Hu − Ho (hardness difference 23 23 19 24 24 27 27 19 34 between center and surface) Hq − Hp (hardness difference 2 2 −2 2 1 4 4 1 6 between 25% and 55% positions) Hs − Hq (hardness difference 2 2 −1 5 3 22 22 8 1 between 55% and 85% positions) (A) − (B) compressive deformation difference (mm) −0.1 −0.1 0 −0.4 −0.5 3.8 3.6 −0.2 0 (A) − (C) compressive deformation difference (mm) 1.8 1.8 1.7 0.9 0.7 7.4 7.3 1.5 1.8 Flight Initial velocity (m/s) 73 73 73 74 74 68 68 73 72 on shots Spin rate (rpm) 2439 2559 2660 2880 3051 2188 2377 2655 2554 with W#1 Launch angle (°) 11 11 11 10.7 10.8 11.7 11.7 11 11 (HS, 50 m/s) Carry (m) 250 249 256 251 251 229 230 247 249 Rating good good NG NG NG NG NG NG good Flight Initial velocity (m/s) 55 55 55 55 55 54 54 55 55 on shots Spin rate (rpm) 6046 6236 6459 7782 7483 4863 4417 6586 6288 with I#6 Carry (m) 157 158 156 152 153 156 157 156 157 (HS, 44 m/s) Rating good NG NG NG NG NG good NG NG Spin rate Spin rate (rpm) 5265 6265 6272 6665 6644 5852 5866 6353 6242 on approach Rating NG good good good good good good good good shots (HS, 21 m/s) Durability Rating good good good NG NG NG NG good good to impact Feel When struck with driver good good good NG NG NG NG good good at impact When struck with iron good good good NG NG NG NG good good

As shown in Table 3 above, the golf balls obtained in the examples of the invention had an excellent flight performance and a low spin rate when struck using a W#1 or an I#6, and moreover had a good spin rate on approach shots, a good durability to impact, and a good feel. By contrast, as shown in Table 4, Comparative Examples 1 to 9 had the following drawbacks.

In Comparative Example 1, the outer cover layer was hard, as a result of which the spin rate on approach shots was low.

In Comparative Example 2, the core hardness profile had a place where the hardness decreases toward the core surface from a position 55% of D from the core center. As a result, a sufficient spin rate-lowering effect was not obtained on shots with a I#6.

In Comparative Example 3, the core hardness profile had a place where the hardness decreases toward the core surface from a position 55% of D from the core center. As a result, a sufficient spin rate-lowering effect was not obtained on shots with a W#1 or a I#6, and the distance on shots with a I#6 was less than satisfactory.

In Comparative Example 4, the core hardness profile had a place where the hardness decreases toward the core surface from a position 55% of D from the core center. As a result, a sufficient spin rate-lowering effect was not obtained on shots with a W#1 or a I#6, the distance on shots with a I#6 was less than satisfactory, the feel of the ball at impact was poor and the durability when repeatedly struck was poor.

In Comparative Example 5, the core hardness profile had a place where the hardness decreases toward the core surface from a position 55% of D from the core center, and the hardness at the core center was high. As a result, a sufficient spin rate-lowering effect was not obtained on shots with a W#1 or a I#6, the distance on shots with a I#6 was less than satisfactory, the feel of the ball at impact was poor and the durability of the ball when repeatedly struck was poor.

In Comparative Example 6, the above (A)−(B) value, which is the difference between the compressive deformations of the core and the ball under specific loads, was too large. As a result, the distance traveled by the ball was less than satisfactory, in addition to which the durability of the ball when repeatedly struck was poor and the feel at impact was poor.

In Comparative Example 7, the above (A)−(B) value, which is the difference between the compressive deformations of the core and the ball under specific loads, was too large. As a result, the distance traveled by the ball on shots with a W#1 was less than satisfactory. In addition, the durability of the ball when repeatedly struck was poor, and the feel at impact was poor.

In Comparative Example 8, the hardness difference between the core center and the core surface, expressed in JIS-C hardness units, was smaller than 20, in addition to which the core center hardness was high. As a result, the spin rate-lowering effects on shots with a W#1 or a I#6 were inadequate and the distance traveled on shots with I#6 was less than satisfactory.

In Comparative Example 9, the core hardness profile had a place where the hardness decreases toward the core surface from a position 55% of D from the core center. As a result, the spin rate-lowering effect on shots with a I#6 was inadequate. 

1. A multi-piece solid golf ball comprising a core, at least one intermediate layer and an outer layer, wherein the core is formed of a single layer or of two or more layers; letting Ho be a JIS-C hardness at a center of the core and Hu be a JIS-C hardness at a surface of the core, the JIS-C hardness Ho is from 40 to 70, the JIS-C hardness Hu is from 75 to 95 and Hu−Ho is at least 20; letting D (mm) be a radius of the core, the core has a hardness profile in which the hardness does not decrease toward the core surface from a position 55% of D from the core center; letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf and (B) be the compressive deformation (mm) of the golf ball when compressed under a final load of 600 kgf from an initial load state of 10 kgf, the value (A)−(B) is in the range of from −0.4 to 2.5; the outer layer is formed primary of a thermoplastic polyurethane material; and the outer layer has a Shore D material hardness of at least 40 and not more than
 50. 2. The multi-piece solid golf ball of claim 1 wherein, letting (A) be the compressive deformation (mm) of the core when compressed under a final load of 450 kgf from an initial load state of 10 kgf, and (C) be the compressive deformation (mm) of the golf ball when compressed under a final load of 450 kgf from an initial load state of 10 kgf, the value (A)−(C) is in the range of from 1 to 4.8.
 3. The multi-piece solid golf ball of claim 1, wherein the core has a diameter of from 30 to 39.2 mm.
 4. The multi-piece solid golf ball of claim 1, wherein the intermediate layer has a Shore D material hardness of at least
 50. 5. The multi-piece solid golf ball of claim 1, wherein the outer layer has a thickness of from 0.3 to 1.5 mm. 